Electronic circuit

ABSTRACT

An electronic circuit formed on an insulating substrate and having thin-film transistors (TFTs) comprising semiconductor layers. The thickness of the semiconductor layers is less than 1500 Å, e.g., between 100 and 750 Å. A first layer consisting mainly of titanium and nitrogen is formed on the semiconductor layer. A second layer consisting aluminum is formed on top of the first layer. The first and second layers are patterned into conductive interconnects. The bottom surface of the second layer is substantially totally in intimate contact with the first layer. The interconnects have good contacts with the semiconductor layer.

This application is a Divisional of Ser. No. 08/636,917, filed Apr. 24,1996; now U.S. Pat. No. 5,804,878, which itself is a continuation ofSer. No. 08/162,357, filed Dec. 7, 1993 (now abandoned).

FIELD OF THE INVENTION

The present invention relates to an electronic circuit which is formedon an insulating substrate and has a thin semiconductor layer ofsilicon, for example, forming thin-film transistors, the thinsemiconductor layer being required to be connected with conductiveinterconnects.

BACKGROUND OF THE INVENTION

Conventional thin-film devices such as insulated-gate FETs use a thinsemiconductor film of silicon as an active layer. This layer is about1500 Å thick. Therefore, where electrodes should be formed on this thinsemiconductor film, satisfactory contacts can be made by bringing ametal such as aluminum into direct and intimate contact with the film,in the same way as in the prior art IC fabrication techniques. In thesecontacts, a silicide such as aluminum silicide is usually formed by achemical reaction between the aluminum and the semiconductor componentsuch as silicon. Since the semiconductor layer is sufficiently thickerthan the silicide layer, no problems take place.

However, researches conducted recently have demonstrated that if thethickness of the active layer is decreased below 1500 Å, for example,between about 100 to 750 Å, then the characteristics of the TFTs areimproved. Where electrodes should be formed on such a thin semiconductorlayer, or an active layer, it has not been possible to make goodcontacts by the prior art techniques, because the thickness of thesilicide layer grows almost up to the thickness of the semiconductorlayer, thus severely deteriorating the electrical characteristics of thecontacts. When a stress such as a voltage is kept applied to thecontacts for a long time, the contacts deteriorate seriously.

In order to improve the characteristics of the TFTS, thermal treatmenteffected below 400° C., typically 200-350° C., within hydrogen ambientis needed after formation of the electrodes on the semiconductor layer.Where the thickness of the semiconductor layer of the TFTs is less than1500 Å, the thermal processing greatly promotes growth of the silicide,leading to deterioration of the characteristics of the TFTs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a reliableelectronic circuit having a semiconductor layer, conductiveinterconnects, and good contacts between the semiconductor layer and theinterconnects, the contacts being capable of withstanding thermalprocessing performed at or above 300° C.

The present invention resides in an electronic circuit which is formedon an insulating substrate and has a semiconductor layer consistingmainly of silicon, the thickness of the semiconductor layer being lessthan 1500 Å, preferably between 100 Å and 750 Å. For example, theinvention is applicable to an electronic circuit having TFTs eachprovided with an active layer having a thickness less than 1500 Å. Theeffects of the present invention become conspicuous as the thickness ofthe semiconductor layer decreases.

In a first embodiment of the invention, the above-describedsemiconductor layer in the form of a thin film is either in intimatecontact with the top surface of the insulating substrate as made ofglass or formed over this substrate via some insulating film. A firstlayer consisting principally of titanium and nitrogen is partially ortotally in intimate contact with the semiconductor layer. A second layerconsisting principally of aluminum is formed on the top surface of thefirst layer. The first and second layers are photolithographicallypatterned into conductive interconnects. The bottom surface of thesecond layer is substantially totally in intimate contact with the firstlayer. It is possible to form a third layer consisting mainly oftitanium and nitrogen on the second layer.

In another embodiment of the invention, the above-describedsemiconductor layer in the form of a thin film is either in intimatecontact with the insulating substrate as made of glass or formed overthis substrate via some insulating film. A first layer containing bothtitanium and silicon is partially or totally in intimate contact withthe semiconductor layer. A second layer consisting chiefly of titaniumand nitrogen is in intimate contact with the top surface of the firstlayer. A third layer consisting principally of aluminum is formed on thetop surface of the second layer. The first through third layers arephotolithographically patterned into conductive interconnects. Ofcourse, other layer may be formed on the third layer.

In a further embodiment of the invention, the above-describedsemiconductor layer in the form of a thin film is either in intimatecontact with the insulating substrate as made of glass or formed overthis substrate via some insulating film. A first layer containing bothtitanium and silicon as main constituents is partially or totally inintimate contact with the semiconductor layer. A second layer consistingchiefly of titanium and nitrogen is in intimate contact with the topsurface of the first layer. A third layer consisting principally ofaluminum is formed on the top surface of the second layer. The firstthrough third layers are photolithographically patterned into conductiveinterconnects. This embodiment is characterized in that the ratio of thetitanium to the nitrogen in the first layer is greater than thetitanium/nitrogen ratio of the second layer.

In any structure of these embodiments, the portions of the thinsemiconductor film with which the first layer is in intimate contactshow an N- or P-type conductivity. Preferably, the dose in theseportions is 1×10¹⁹ to 1×10²⁰ /cm². The impurity may be introduced by awell-known ion implantation method or plasma doping method. Where suchimpurity ions are accelerated to a high energy and introduced, the doseis preferably between 0.8×10¹⁵ and 1×10¹⁷ /cm². Also, a laser dopingmethod using laser irradiation within an ambient of an impurity gas maybe utilized. This method is described in Japanese Patent applicationSer. No. 283981/1991, filed Oct. 4, 1991, and No. 290719/1991, filedOct. 8, 1991. Preferably, the sheet resistance of these portions is lessthan 1 kΩ/cm².

Elements which can be added to the semiconductor layer are phosphorus,boron, arsenic, and others. Those portions of the semiconductor layerwhich are in contact with the conductive interconnects may be parts ofdoped regions such as the source and drain regions of the TFTS.Preferably, the sheet resistance of the semiconductor layer is less than500Ω/.

A silicon oxide layer may be in intimate contact with the bottom surfaceof the thin semiconductor layer. In this structure, the silicon oxidefilm may contain the same impurity as the impurity contained in thesemiconductor layer.

In the first layer of the above-described first embodiment, the ratio ofthe titanium to the nitrogen contained as main constituents may differaccording to the thickness. Besides titanium and nitrogen, otherelements such as silicon and oxygen can be contained as mainconstituents. For example, that portion of the first layer which isclose to the semiconductor layer may consist principally of titanium andsilicon. That portion of the first layer which is close to the secondlayer may consist mainly of titanium and nitrogen. For instance, theratio of nitrogen to titanium may be set close to a stoichiometric ratio(exceeding 0.8). In the intermediate region, the constituents may bemade to vary continuously.

Generally, a stoichiometric material (titanium nitride) containingnitrogen and titanium has excellent barrier characteristics and preventsdiffusion of aluminum and silicon. However, the material shows a highcontact resistance with silicon. Therefore, it is not desired to usesuch a material directly for formation of contacts. On the other hand, astoichiometric material (titanium silicide) containing titanium andsilicon exhibits a low contact resistance with the semiconductor layerconsisting mainly of silicon. This is advantageous to form Ohmiccontacts. However, aluminum tends to easily diffuse. For example, thealuminum of the second layer diffuses through the first layer, thusforming aluminum silicide in the semiconductor layer.

The complex layer structure described above has been formed to solvethese problems. In particular, that portion which is in contact with thesecond layer is made of substantially stoichiometric titanium nitrideand hence the titanium nitride has excellent barrier characteristics.This prevents the aluminum of the second layer from diffusing into thefirst layer. The portion in contact with the semiconductor layer is madeof substantially stoichiometric titanium silicide. Thus, good Ohmiccontacts can be derived.

When a film of titanium silicide is formed, it is not necessary tointentionally add silicon. Titanium reacts with the silicon contained inthe semiconductor layer. As a result, titanium silicide is automaticallyformed. For example, therefore, similar effects can be produced bydepositing titanium containing less nitrogen onto the portion close tothe semiconductor layer and depositing titanium containing more nitrogenonto the portion close to the second layer.

In either case, when the whole first layer is viewed, it consists mainlyof titanium and nitrogen. Preferably, the ratio of nitrogen to titaniumin the first layer is 0.5 to 1.2. This material containing titanium andnitrogen as main constituents can make Ohmic contacts with a conductiveoxide such as indium tin oxide, zinc oxide, and nickel oxide. Wherealuminum and such a conductive oxide together form a junction, a thicklayer of aluminum oxide is formed at this junction, and it is impossibleto have good contacts. In the prior art techniques, a chromium layer hasbeen formed between aluminum and a conductive oxide. Since the chromiumis poisonous, alternative materials have been sought for. Materials usedin the present invention and consisting mainly of titanium and nitrogenare excellent also in this respect.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(D) are cross sections of a circuit using TFTs accordingto the invention, illustrating the sequence in which the circuit isfabricated;

FIG. 2(A) is a vertical cross section of an electronic circuit accordingto the invention;

FIG. 2(B) is a top view of another electronic circuit according to theinvention;

FIG. 3 is a graph illustrating the characteristic curve a of TFTsfabricated according to the invention, as well as the characteristiccurve b of TFTs fabricated by the prior art method;

FIGS. 4(A) and 4(B) are photographs of contact holes in TFTs;

FIG. 5(A) is a schematic view illustrating the contact hole shown inFIG. 4(A);

FIG. 5(B) is a schematic view illustrating the contact hole shown inFIG. 4(B);

FIG. 6 is a schematic cross section of a device comprising a pluralityof TFTs according to the invention, the TFTs being formed on asubstrate;

FIGS. 7(A) to 7(H) are cross sections of a TFT according to theinvention, illustrating the sequence in which the TFT is fabricated; and

FIGS. 8(A) to 8(C) are cross sections of TFTs according to theinvention, illustrating the contacts of the source or drain.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1

Example 1 is shown in FIGS. 1, (A)-(D), and 2, (A)-(B). FIGS. 1,(A)-(D), illustrate the sequence in which an electronic circuit havingTFTs is fabricated. Description of conventional steps is omitted. First,silicon oxide was deposited as a silicon oxide film 2 forming a basefilm on a glass substrate 1 made of Corning 7059. An amorphous siliconfilm 3 having a thickness of 500 to 1500 Å, preferably 500 to 750 Å, wasformed on the silicon oxide film 2. A protective layer 4 was formed onthe amorphous silicon film 3. The laminate was annealed at 450 to 600°C. for 12 to 48 hours to crystallize the amorphous silicon film. Ofcourse, laser annealing or other similar means can be used for thecrystallization (FIG. 1(A)).

The silicon film was photolithographically patterned into island-shapedsemiconductor regions 5. A silicon oxide film 6 having a thickness of500 to 1500 Å, preferably 800 to 1000 Å, was formed on the semiconductorregions 5 to form a gate oxide film. Then, gate interconnects andelectrodes 7 were fabricated from aluminum. The aluminum interconnectsand electrodes 7 were anodized to form an aluminum oxide coating aroundthe interconnects and electrodes 7. This technique which makes use ofanodization for top-gate TFTs in this way is described in JapanesePatent application Ser. No. 38637/1992, filed Jan. 24, 1992. Of course,the gate electrodes can be made of silicon, titanium, tantalum,tungsten, molybdenum, or other material. Thereafter, using the gateelectrodes as a mask, an impurity such as phosphorus was implanted byplasma doping or other method to form doped silicon regions 8 alignedwith the gate electrodes 7. Then, the doped regions 8 wererecrystallized by thermal annealing, laser annealing, or other method toform source and drain regions of TFTs (FIG. 1(B)).

Then, silicon oxide was deposited as an interlayer insulator 9. Aconductive transparent oxide such as ITO (indium tin oxide) wasdeposited. The ITO film was photolithographically patterned into pixelelectrodes 10 of an active-matrix liquid crystal display. Contact holeswere formed in the interlayer insulator 9 to expose parts of the dopedregions, or the source and drain regions. A first layer consistingmainly of titanium and nitrogen was formed by sputtering. Also, a secondlayer made of aluminum was formed by sputtering in the manner describedbelow.

A target made of titanium was placed in a sputter chamber. Films wereformed within argon ambient. The sputtering pressure was 1 to 10 mtorr.First, a layer having titanium as its main constituent but containinglittle nitrogen was formed up to a thickness of 50 to 500 Å. Besidesargon, nitrogen was introduced into the sputter chamber. Within thisambient, a film was formed by sputtering. As a result, a layer ofsubstantially stoichiometric titanium nitride and having a thickness of200 to 1000 Å was formed. At this time, the percentage of nitrogen inthe sputtering ambient was in excess of 40%. It is to be noted that thedeposition rate by sputtering is affected greatly by the partialpressure of the nitrogen as well as by the sputtering pressure. Forexample, the deposition rate within an ambient consisting only of argonis generally 3 to 5 times as high as the deposition rate within anambient containing more than 20% nitrogen. With respect to thesputtering ambient, ammonia, hydradine, or other substance can be usedinstead of nitrogen. It is known that the resistivity of the producedfilm varies, depending on the partial pressure of nitrogen duringsputtering. Since the film is used to form conductive interconnects, alower resistivity is desired. For this purpose, of course, an optimumpartial pressure of nitrogen is adopted. For example, an ambientcontaining 100% nitrogen produces a lower resistivity than theresistivity obtained within an ambient containing 40% nitrogen. Typicalresistivities were between 50 and 300 μΩ cm.

In the steps described above, if the titanium layer which was formedfirst and contained little nitrogen was too thick, reaction with theunderlying layer occurred. This made it impossible to obtain goodcontacts. Our research has demonstrated that if the titanium layer isthinner than the semiconductor layer, then favorable results areobtained.

After forming a first layer 11 in this way, aluminum was sputtered toform a second layer 12 containing 1% silicon. The thickness of thissecond layer was 2000 to 5000 Å. These layers were photolithographicallypatterned. More specifically, the second layer of aluminum was etchedwith an etchant such as a mixture acid of phosphoric acid, acetic acid,and nitric acid. Subsequently, the first layer was etched with bufferedhydrofluoric acid or nitrous acid while leaving behind the photoresiston the aluminum film. At this time, the interlayer insulator wasdeteriorated by overetching. The etching process might also be carriedout by etching the first layer with a mixture of aqueous solution ofhydrogen peroxide (H₂ O₂) and aqueous ammonia (NH₃ OH), using thealuminum layer selectively left first as a mask. In this case, theinterlayer insulator was not affected. However, organic materials suchas the photoresist were oxidized.

The above-described etching step can be a dry etching process. If carbontetrachloride (CCl₄) is used as an etching gas, the second and firstlayers can be continuously etched without adversely affecting thesilicon oxide. In this way, conductive interconnects extending from thedoped regions were formed. Then, the laminate was annealed at 300° C.within an ambient of hydrogen, thus completing TFTs.

The circuit fabricated in this way had portions which were required tobe connected to the outside. FIG. 2(A) shows the manner in whichconductive interconnects 19 for connection with the outside extend froman integrated circuit 18 toward substrate peripheral portions, thecircuit 18 being formed on a substrate 17. This electronic circuit maysometimes make electrical contact by mechanical means such as contactfixtures (e.g., sockets) in regions 20 surrounded by the broken line.

In a liquid crystal display as shown in FIG. 2(B), circuits 22-24activate an active-matrix region 25 on a substrate 21. In order tosupply electric power and signals to the circuits 22-24, electricalcontacts are made in regions 27 surrounded by the broken lines.Connections made by wire bonding function permanently and are highlyreliable. However, considerable labor is needed to fabricate theconnections. Especially, this method is not suited for connection ofnumerous terminals. Therefore, use of mechanical contacts may sometimesbe more advantageous.

In this case, however, it is necessary that the surfaces of theconductive interconnects at the contacts be sufficiently strong and thatthe underlying layer adhere well to the conductive interconnects.Aluminum is not suitable for these purposes. A material consistingmainly of titanium adheres well to silicon, silicon oxide, aluminum, andother similar materials. Also, the hardness of the coating of thismaterial is high. Hence, this material is adequate. It is possible thatnitrogen be not contained at all. Also, a maximum amount of nitrogen upto its stoichiometric ratio can be contained. In the present example, ofthe first layer 11, only the contacts were etched to expose the secondlayer. In the present example, those portions of the first layer whichwere in contact with the second layer were made of stoichiometrictitanium nitride. Contact fixtures 13 were pressed against the exposedportions of the titanium nitride to form contacts (FIG. 1(C)).

Alternatively, as shown in FIG. 1(D), a second layer 15 is formed on afirst layer 14. A third layer 16 of titanium nitride is formed on thesecond layer 15. Contact fixtures may be brought into contact with thisthird layer. In this case, it is not necessary to partially etch thesecond layer as shown in FIG. 1(C). Hence, the patterning step can beomitted. Furthermore, as shown in FIG. 1(E), a layer consisting mainlyof nitrogen and titanium according to the present invention is firstphotolithographically patterned into conductive interconnects, and thenan ITO film is formed. In either case, in the present example, the ITOfilm is made of a material consisting principally of nitrogen andtitanium. Consequently, good contacts can be obtained. The material ofthe film is not restricted to ITO. Rather, various other conductiveoxides may also be used.

The V_(D) -I_(D) characteristic of the TFTs obtained in this way isshown as curve a in FIG. 3. For reference, the V_(D) -I_(D)characteristic of TFTs having conventional Al/Si contacts is shown ascurve b in FIG. 3. A kink is observed close to V_(D) =0 on the curve bof the TFTs manufactured by the prior art method. Their contactresistances do not make Ohmic contact. On the other hand, suchabnormality is not observed on the curve a of the TFTs manufacturedaccording to the present invention, and normal MOS FET characteristicsare exhibited.

FIGS. 4(A) and 4(B) are photographs demonstrating that alloying (i.e.,formation of a silicide) of the materials of the conductiveinterconnects extending from the TFTS, i.e., aluminum, with the N-typesilicon in the source and drain regions is suppressed under the sameconditions as in Example 1. The regions shown in the photographs ofFIGS. 4(A) and 4(B) are schematically shown in FIGS. 5(A) and 5(B),respectively. A rectangular region seen at the center of each photographis a contact hole. After formation of the contact, the laminate wasannealed at 300° C. for 30 minutes. Where no titanium nitride existedbetween the silicon and the aluminum as shown in FIG. 4(A), a largeamount of silicide (specks) was produced at the contacts. Where a filmof titanium nitride having a thickness of 1000 Å existed as shown inFIG. 4(B), any deterioration was not observed at all.

EXAMPLE 2

The present example is described by referring to FIG. 1, (A)-(D), whichconceptually illustrate the sequence in which an electronic circuithaving TFTs is fabricated. Conventional steps are not described herein.First, silicon oxide was deposited as a silicon oxide base film 2 on aglass substrate 1. An amorphous silicon film 3 having a thickness of 100to 1500 Å, preferably 100 to 750 Å, was formed on the silicon oxide film2. A protective layer 4 was formed on the amorphous silicon film 3. Thelaminate was annealed at 450 to 600° C. for 12 to 48 hours tocrystallize the amorphous silicon film. Of course, laser annealing orother similar means can also be used for the crystallization (FIG.1(A)).

The silicon film was photolithographically patterned into island-shapedsemiconductor regions 5. A silicon oxide film 6 having a thickness of500 to 1500 Å, preferably 800 to 1000 Å, was formed on the semiconductorregions 5 to form a gate oxide film. Then, gate interconnects andelectrodes 7 were fabricated from aluminum. The aluminum interconnectsand electrodes 7 were anodized to form an aluminum oxide coating aroundthe interconnects and electrodes 7. Thereafter, using the gateelectrodes as a mask, an impurity such as phosphorus was introduced byion implantation or other method to form doped silicon regions 8 alignedwith the gate electrodes 7. The dose, the accelerating voltage, and thethickness of the gate oxide film were so set that the dose was 0.8 to4×10¹⁵ /cm² and that the dopant density was 1×10¹⁹ to 1×10²¹ /cm³. Then,the doped regions 8 were recrystallized by thermal annealing, laserannealing, or other method to form source and drain regions of TFTs(FIG. 1(B)).

Then, silicon oxide was deposited as an interlayer insulator 9, followedby deposition of ITO. The ITO film was photolithographically patternedinto pixel electrodes 10 of an active-matrix liquid crystal display.Contact holes were formed in the interlayer insulator 9 to expose partsof the doped regions, or the source and drain regions. A first layerconsisting mainly of titanium and nitrogen was formed by sputtering.Also, a second layer made of aluminum was formed by sputtering in themanner described below.

A target made of titanium was placed in a sputter chamber. Films wereformed within an ambient consisting of argon and nitrogen. The ratio ofthe partial pressure of the argon to the partial pressure of thenitrogen was less than 0.3, for example 0.25. The sputtering pressurewas 3 mtorr. A DC current of 4.5 A was passed. The flow rate of theargon was 24 SCCM. The flow rate of the nitrogen was 6 SCCM. The firstlayer had a lower layer containing less nitrogen, the lower layer havinga thickness of 100 Å. The film formed in this way showed sufficientlysmall contact resistance with the silicon and ITO.

Then, the percentage of the ambient within the sputter chamber wasincreased such that the ratio of the partial pressure of the argon tothe partial pressure of the nitrogen was in excess of 0.3, forexample 1. Within this ambient, a film was formed by sputtering. Thesputtering pressure and the DC current were maintained at 3 mtorr and4.5 Å, respectively. The flow rates of the argon and the nitrogen wereset to 15 SCCM. By the steps described above, an upper layer (having athickness of 900 Å) of the first layer was formed. The film formed inthis manner showed a large contact resistance with the silicon and socould not be used as contacts. However, this film could be patternedinto conductive interconnections without difficulty in the presentexample. It is to be noted that the deposition rate by sputtering isaffected greatly by the partial pressure of the nitrogen as well as bythe sputtering pressure. For example, where the ratio of argon tonitrogen was 4:1, the deposition rate was 100 to 120 Å/min. Where theratio of argon to nitrogen was 1:1, the deposition rate was 30 to 40Å/min.

After forming the first layer 11 in this way, aluminum was sputtered toform a second layer 12 containing 1% silicon. The thickness of thissecond layer was 2000 to 5000 Å. These layers were photolithographicallypatterned. More specifically, the second layer of aluminum was etchedwith an etchant such as a mixture acid of phosphoric acid, acetic acid,and nitric acid. Subsequently, the first layer was etched with a mixtureliquid of aqueous solution of hydrogen peroxide (H₂ O₂) and aqueousammonia (NH₃ OH) while leaving behind the photoresist on the aluminumfilm. Since this etchant oxidizes organic substances, it follows that afinal cleaning of organic substances is simultaneously done. In thisway, conductive interconnects extending from the doped regions wereformed. Then, the laminate was annealed at 300° C. within an ambient ofhydrogen, thus completing TFTS. In the present example, of the firstlayer 11, only the contacts were etched, thus exposing the second layer.Contact fixtures 13 were pressed against the exposed portions of thefirst layer to form contacts (FIG. 1(C)).

EXAMPLE 3

The present example is shown in FIGS. 7, (A)-(H). First, silicon oxidewas deposited as a silicon oxide film 202 on a glass substrate 201 madeof Corning 7059. The silicon oxide film formed a base film and had athickness of 1000 to 3000 Å. The substrate measured 300 mm×400 mm or 100mm×100 mm. To form this oxide film, sputtering was effected withinoxygen ambient. To make mass production more efficient, TEOS may bedecomposed and deposited by plasma CVD.

Then, amorphous silicon was deposited as a film having a thickness of300 to 5000 Å, preferably 500 to 1000 Å, by plasma CVD or LPCVD. Thisfilm was allowed to stand within an oxidizing ambient at 550 to 600° C.for 24 hours to crystallize the film. This step may also be carried outby laser irradiation. The crystallized silicon film wasphotolithographically patterned into island-shaped regions 203. Asilicon oxide film 104 having a thickness of 700 to 1500 Å was formed bysputtering techniques.

An aluminum film having a thickness of 1000 Å to 3 μm was then formed byelectron-beam evaporation or sputtering. This aluminum film contained 1%by weight of silicon or 0.1 to 0.3% by weight of scandium. A film ofphotoresist, such as OFPR800/30 cp prepared by TOKYO OHKA KOGYO CO.,LTD., was formed by spin coating. If a film of aluminum oxide having athickness of 100 to 1000 Å was formed by anodization before theformation of the photoresist film, then the aluminum film adhered wellto the photoresist film. Also, leakage of electric current from thephotoresist layer was suppressed. This was effective in forming porousanodized oxide in a subsequent anodization step. Thereafter, thephotoresist film and the aluminum film were photolithographicallypatterned and etched to form gate electrodes 205 and a masking film 200(FIG. 7(A)).

The gate electrodes 205 were anodized by passing an electric currentthrough an electrolytic solution to form an anodic oxide film 206 havinga thickness of 3000 to 6000 Å, for example 5000 Å. The anodization stepwas carried out, using an acidic solution of 3 to 20% of citric acid,nitric acid, phosphoric acid, chromic acid, sulfuric acid, or otheracid, and by applying a constant voltage of 10 to 30 V to the gateelectrodes. In the present example, a voltage of 10 V was applied to thegate electrodes in oxalic acid at 30° C. for 20 to 40 minutes for anodicoxidation. The thickness of the anodic oxide film was controlled by theanodization time (FIG. 7(B)).

Subsequently, the silicon oxide film 104 was etched by dry etchingtechniques. In this etching step, either plasma mode of isotropicetching or reactive ion etching mode of anisotropic etching can be used.However, it is important that the active layer be etched not deeply bysetting large the selection ratio of the silicon to the silicon oxide.For example, if CF₄ is used as an etching gas, the anodic oxide film isnot etched; only the silicon oxide film 104 is etched. The silicon oxidefilm 204 located under the porous anodic oxide film 206 was not etchedbut left behind (FIG. 7(C)).

An electric current was again supplied to each gate electrode within anelectrolytic solution. At this time, ethylene glycol solution containing3 to 10% tartaric acid, boric acid, or nitric acid was used. When thetemperature of the solution was lower than room temperature, or about10° C., a good oxide film was obtained. In this way, a barrier-typeanodic oxide film 207 was formed on the top and side surfaces of thegate electrodes. The thickness of the anodic oxide film 207 was inproportion to the applied voltage. When the applied voltage was 150 V,the thickness of the formed anodic oxide film was 2000 Å. In the presentexample, the voltage was increased to 80-150 V. The value of the voltagewas determined according to the required thickness of the anodic oxidefilm 207 (FIG. 7(D)).

Using the barrier-type anodic oxide film 207 as a mask, the porousanodic oxide film 206 was etched away. Then, using the gate electrodeportions 205 and 207 and the gate-insulating film 204 as masks, animpurity was implanted by ion doping to form low-resistivity dopedregions 208, 211 and high-resistivity doped regions 209, 210. The dosewas 1 to 5×10¹⁴ /cm². The accelerating voltage was 39 to 90 kV.Phosphorus was used as the impurity (FIG. 7(E)).

An appropriate metal such as titanium, nickel, molybdenum, tungsten,platinum, or palladium was sputtered over the whole surface. Forinstance, a titanium film 212 having a thickness of 50 to 500 Å wasformed over the whole surface. As a result, the metal film, the titaniumfilm 212 in this example, was in intimate contact with thelow-resistivity doped regions 208 and 211 (FIG. 7(F)).

Laser radiation emitted from a KrF excimer laser having a wavelength of248 nm and a pulse width of 20 nsec was illuminated to activate theimplanted impurity and to cause the metal film, or the titanium film, toreact with the active layer, thus forming regions 213 and 214 of a metalsilicide, or titanium silicide. The energy density of the laserradiation was 200 to 400 mJ/cm², preferably 250 to 300 mJ/cm². When thelaser light was illuminated, if the substrate was heated to 200 to 500°C., then the peeling of the titanium film could be suppressed.

In the present example, an excimer laser was employed as describedabove. Of course, other laser can be used. Preferably, the used laser isa pulsed laser. If a CW laser is used, the illumination time is long andso the illuminated object expands due to heat. As a result, the objectmay peel off.

Usable pulsed lasers include infrared lasers such as an Nd:YAG laser(preferably Q-switched laser), visible light lasers such as thoseutilizing second-harmonic generation, and various UV lasers usingexcimers such as KrF, XeCl, and ArF. Where laser light is illuminatedfrom above the metal film, it is necessary that the wavelength of thelaser light be so selected that the light is not reflected from themetal film. However, where the metal film is quite thin, almost noproblems take place. The laser light may also be illuminated from theside of the substrate. In this case, such laser light which istransmitted through the underlying silicon semiconductor layer isrequired to be selected.

The anneal can be a lamp anneal using illumination of visible light ornear infrared light. Where a lamp anneal is conducted, light isilluminated in such a way that the surface of the illuminated objectreaches about 600 to 1000° C. Where the temperature is 600° C., theillumination is continued for several minutes. Where the temperature is1000° C., the illumination is carried out for tens of seconds. An annealusing infrared light such as infrared light of 1.2 μm is quiteadvantageous for the following reasons. The near infrared light isabsorbed selectively by the silicon semiconductor layer and, therefore,the glass substrate is not heated very much. By setting eachillumination time short, the substrate is heated to a less extent.

Then, the titanium film was etched by an etchant consisting of hydrogenperoxide, ammonia, and water at a rate of 5:2:2. The exposed layer andthose portions of the titanium layer which were not contacted (e.g., thetitanium film existing on the gate-insulating film 204 and on the anodicoxide film 207) were left in metal state. These portions could beremoved by this etching. Since none of the titanium nitride films 213and 214 were etched, they could be left behind (FIG. 7(G)).

Finally, as shown in FIG. 7(H), a silicon oxide film having a thicknessof 2000 Å to 1 μm (e.g., 3000 Å) was formed as an interlayer insulator217 over the whole surface by CVD. Contact holes were formed in thesource and drain electrodes of the TFTs. Aluminum interconnects andelectrodes 218 and 219 having thicknesses of 200 Å to 1 μm (e.g., 5000Å) were formed. In the present example, the portions with which thealuminum interconnects were in contact were made of titanium silicide.The stability at the interface with the aluminum is improved over thecase of silicon. Hence, reliable contacts were obtained. If a barriermetal such as titanium nitride was deposited between the aluminumelectrodes 218, 219 and the silicide regions 213, 214, the reliabilitycould be improved further. In the present example, the sheet resistanceof the silicide regions was 10 to 50 Ω/cm². The sheet resistance of thehigh-resistivity regions 209 and 210 was 10 to 100 kΩ/cm². As a result,TFTs which had good frequency characteristics and suffered from less hotcarrier deterioration at high drain voltages could be fabricated. In thepresent example, the low-resistivity doped region 211 could be madesubstantially coincident with the metal silicide regions.

FIG. 6 illustrates an example of fabrication of plural TFTs on asubstrate, by the method illustrated in FIGS. 7, (A)-(H). In thisexample, three thin-film transistors TFT1-TFT3 were formed. The TFT1 andTFT2 were used as driver TFTs and took the form of CMOS devices. In thepresent example, these TFTs were built as inverters. Oxide layers 505and 506 corresponding to the anodic oxide film 207 shown in FIGS. 7,(A)-(H), had a small thickness of 200 to 1000 Å, for example 500 Å.These oxide layers slightly overlapped the underlying layer. The TFT3was used as a pixel TFT. An anodic oxide film 507 had a large value of2000 Å and took an offset state, thus suppressing leakage current. Oneof the source/drain electrodes of the TFT3 was connected with the pixelelectrode 508 of ITO. In order that the anodic oxide films had differentthicknesses, they were separated to permit the voltages applied to thegate electrodes of the TFTs to be controlled independently. The TFT1 andTFT3 were n-channel thin-film transistors, while the TFT2 was ap-channel thin-film transistor.

In the present example, the step for forming the titanium film wascarried out after the ion doping step. This sequence may be reversed. Inthis case, since the titanium film coats the whole underlying layer whenions are illuminated, abnormal charging, or charge-up, which would beproduced on the substrate, is effectively prevented. As a modifiedexample, after ion doping, a laser annealing step is carried out. Then,a titanium film is formed, and a titanium silicide film is formed bylaser illumination or thermal annealing.

The contacts of the source or drain electrodes of the novel TFTs maytake the structure shown in FIGS. 8(A)-8(C). Shown in these figures area glass substrate 1, an insulating film 6, a source or drain 8, aninterlayer insulating film 9, a titanium silicide region 301, a titaniumnitride layer 302, an aluminum layer 303, a titanium nitride layer 304,a titanium layer 305, and a titanium nitride layer 306.

In the present invention, thin source, drain, or other doped regions, ofTFTs can have good contacts, which are highly reliable and henceeffective in enhancing the reliability of the whole electronic circuit.In this way, the invention is industrially advantageous.

What is claimed is:
 1. An electronic circuit comprising:a semiconductorfilm having a thickness less than 1500 Å and comprising silicon; asource and drain provided in said semiconductor film; a channel providedin said semiconductor film between said source and drain; a gateelectrode provided adjacent to said channel with a gate insulating filmtherebetween; and an interconnect being in contact with at least one ofsaid source and drain and comprising a first layer comprising titanium,a second layer comprising aluminum, and a third layer comprisingtitanium, said second layer being provided between said first layer andsaid third layer, wherein said semiconductor film is in contact withsaid first layer at a region thereof which is provided in at least oneof said source and drain and which contains therein an element selectedfrom the group consisting of phosphorous, arsenic and boron at aconcentration of 1×10¹⁹ to 1×10²¹ /cm³.
 2. The circuit of claim 1wherein nitrogen and titanium as constituents of said first layer aremade to vary continuously.
 3. The circuit of claim 1 wherein saidsemiconductor film has a sheet resistance less than 500 Ω/square.
 4. Thecircuit of claim 1 wherein said first layer has a thickness of 200 to1000 Å.
 5. An electronic circuit comprising:a semiconductor film havinga thickness less than 1500 Å and comprising silicon; a source and drainprovided in said semiconductor film; a channel provided in saidsemiconductor film between said source and drain; a gate electrodeprovided adjacent to said channel with a gate insulating filmtherebetween; and an interconnect being in contact with at least one ofsaid source and drain and comprising a first layer comprising titanium,a second layer comprising aluminum, and a third layer comprisingtitanium, said second layer being provided between said first layer andsaid third layer, wherein said semiconductor film is in contact withsaid first layer at a region thereof which is provided in at least oneof said source and drain and which contains therein an element selectedfrom the group consisting of phosphorous, arsenic and boron at aconcentration of 1×10¹⁹ to 1×10²¹ /cm³.
 6. The circuit of claim 5wherein said semiconductor film has a sheet resistance less than 500Ω/square.
 7. The circuit of claim 5 wherein said first layer has athickness of 200 to 1000 Å.